The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 28

where C is the constant which depends on the nature of the bonding between A
and B.
Thus, the electronegativity of atoms A and B in the molecule has the form:

χ A (in AB) = C(Z A + δ)/r 1 (5)

and,

χ B (in AB) = C(Z B - δ )/r 2 (6)

According to Sanderson's electronegativity equalization principle [11], electro-
negativity of bonded atoms in a molecule must be equal to each other.
Thus,
χ AB = χ A (in AB) = χ B (in AB) = C(Z A + δ)/r 1 = C(Z B - δ) /r 2 = C(Z A + Z B )/R AB (7)

or, χ AB = (χ A r A + χ B r B )/(r 1 + r 2 ) = (χ A R AA + χ B R BB )/2R AB (8)

Huheey [22] pointed out that the assumption of electronegativity equalization
ignores energies arising from electrostatic (ion-ion) interaction [23] and changes in
overlap [24]. These are true for extremely ionic bonds, but for the bonds which are
predominantly covalent in nature the errors incurred are small. The errors resulting
from neglect of changes in electrostatic and overlap terms have opposing effects and
tend to cancel each other; both approach zero as δ approaches zero.

JUSTIFICATION OF THE REACTION SURFACE IN TERMS OF
ELECTRONEGATIVITY
Sekhon [25] demonstrated the use of the equalized electronegativity as a guide to the
outcome of metathesis reactions in inorganic and organic chemistry. He had examined
some examples of the exchange reactions of the type AB + CD = AD + BC and con-
cluded that an exchange reaction proceeds from the left to right if the total sum of the
equalized electronegativity of the products is greater than that of the reactants, that is,

/

/

" ,

n

1 |

( )

m

1 |

( )

-ΔE Reac ≈ ½

- (9)

Pr oduct reac tan t j

i = i j

=

Or simply,

-ΔE Reac = Δ χ (10)

where n and m are the number of products and reactants respectively, and

/

/

" ,

n

1 |

( )

m

1 |

( )

Δ χ = ½ - (11)

Pr oduct

reac tan t j

i i

= j =

ELECTRONEGATIVITY AND MOLECULAR ORBITAL THEORY
Coulson [26] introduced the Linear Combination of Atomic Orbital (LCAO) approxi-
mation and proposed the LCAO-MO theory which introduced two important quantities:

1. the Coulomb integral, α r = ∫Φ r Ĥ Φ r dτ, (12)

The Coulomb integral α r , measures the energy of an electron when confi ned to the
atom r within the molecule
and,